1. Field of the Invention
The present invention relates to an antiskid brake controller for preventing wheels from being locked to a road surface on which a vehicle travels, and more specifically, to an antiskid brake controller capable of maintaining high reliability to a difference of road surface friction factors without the use of a road surface sensor.
2. Description of the Related Art
Conventionally, there have been well known antiskid brake controllers which avoid a wheel-locked-state by presumingly calculating a vehicle speed and wheel decelerations based on the wheel speeds of respective wheels when a brake is applied and reducing a braking pressure when necessary. These controllers are generally called ABS (antiskid brake system).
In this type of the controllers, since the threshold values (corresponding to a locked state) of wheel decelerations are set in accordance with road surface states (road surface friction factors) and each time the wheel decelerations exceed the threshold values, a braking pressure is reduced to thereby obtain a maximum braking force within a range in which no wheel-locked-state is caused, it is preferable to set optimum threshold values in accordance with the road states.
FIG. 14 is a block diagram showing the schematic arrangement of a conventional antiskid brake controller disclosed in, for example, Japanese Unexamined Patent Publication No. 8-239024, FIG. 15 is a view specifically showing the arrangement of a hydraulic passage in the vicinity of actuators in FIG. 14 and FIG. 16 is a view showing the arrangement of the actuator in FIG. 14 in more detail paying attention to a wheel.
In the respective drawings, four wheels 1a-1d for driving a vehicle are composed of the front wheels 1a and 1b as driving wheels and rear wheels 1c and 1d as non-driving wheels.
Wheel speed sensors (wheel speed sensing means) 2a-2d for independently detecting the rotational speeds of the respective wheels 1a-1d as wheel speed signals Va-Vd are composed of an electromagnetic pickup type speed sensor or a photoelectric transducing type speed sensor.
The wheel speed sensor 2a mounted in the vicinity of the wheel 1a creates the wheel speed signal Va in accordance with the rotation of the wheel 1a and the wheel speed sensors 2b-2d mounted in the vicinity of the respective wheels 1b-1d create the wheel speed signals Vb-Vd in accordance with the rotations of the wheels 1b-1d likewise.
In FIG. 15, the driving wheels, that is, the front wheels 1a and 1b are coupled with an engine 6 through a driving shaft 4 and a differential mechanism 5, whereas the non-driving wheels, that is, the rear wheels 1c and 1d are not coupled with the engine 6.
Braking units 7a-7d constituting the braking means of the vehicle are composed of wheel cylinders which are pressed against the respective wheels 1a-1d in accordance with braking pressures Pa-Pd and individually disposed to the respective wheels 1a-1d.
A master cylinder 9 is coupled with a brake pedal 8 which is actuated by the driver when the brake is applied to create a braking pressure (hydraulic pressure) in response to an amount of depression of the brake pedal 8.
The master cylinder 9 has actuators 10a-10d which are coupled therewith through a hydraulic passage and composed of a hydraulic unit including electromagnetic solenoids.
The actuators 10a-10d adjust the braking pressure created by the master cylinder 9 in accordance with control signals Ca-Cd (to be described later) and individually supply the thus adjusted braking pressures to the respective brake units 7a-7d.
With this operation, the brake units 7a-7d generate braking forces to the respective wheels 1a-1d in accordance with the amount of actuation of the brake pedal 8 and in response to the control signals Ca-Cd.
In FIG. 14, an ECU (electronic control unit) 11 mounted on the vehicle constitutes the main body of the antiskid brake controller and includes waveform shaping/amplifying circuits 20a-20d , a power supply circuit 22, a microcomputer 23, actuator driving circuits 24a-24d and a motor relay driving circuit 25.
The microcomputer 23 in the ECU 11 includes a CPU 23a for executing various types of calculations and a RAM 23b and a ROM 23c which belong to the CPU 23a.
The ECU 11 constitutes wheel deceleration calculation means which individually calculates wheel decelerations corresponding to the locking tendencies of the respective wheels 1a-1d based on the differential waveforms of wheel speeds Vwa-Vwd obtained from the respective wheel speed signals Va-Vd when the brake is applied.
The ECU 11 further constitutes control amount calculation means for braking force adjustment means which is composed of the actuators 10a-10d, a motor 15 and a motor relay 16. The control amount calculation means executes the calculation for the prevention of the locking tendencies (antiskid control) based on the respective wheel decelerations and adjusts the braking pressures Pa-Pd to the respective wheels 1a-1d by creating the control signals Ca-Cd to the actuators 10a-10d and a control signal CM to the motor relay 16.
The braking force adjustment means adjusts the braking pressures Pa-Pd to the respective wheels 1a-1d in response to the actuation of the brake pedal 8 and based on the control signals Ca-Cd and CM.
The respective waveform shaping/amplifying circuits 20a-20d convert the respective wheel speed signals Va-Vd into pulse signals suitable to calculation and inputs them to the microcomputer 23. The microcomputer 23 calculates the wheel speeds Vwa-Vwd of the respective wheels 1a-1d from the wheel speed signals Va-Vd and uses them to calculate the control signals Ca-Cd.
The power supply circuit 22 supplies a constant voltage to the microcomputer 23 when an ignition switch 27 is turned ON.
The respective actuator driving circuits 24a-24d individually output the control signals Ca-Cd to the electromagnetic solenoids of the respective actuators 10a-10d in response to the control command from the microcomputer 23.
The motor 15 constituting a braking pressure adjusting pump is energized through the normally open contact 16a of the motor relay 16 in response to the control signal CM from the ECU 11 to thereby adjust the braking pressure Pa-Pd in relation to the respective actuators 10a-10d.
The motor relay 16 includes a coil 16b for closing the normally open contact 16a in response to the control signal CM.
The motor 15 and the motor relay 16 constitute the braking force adjustment means for adjusting the braking pressures Pa-Pd to the respective wheels 1a-1d in response to the actuation of the brake together with the actuators 10a-10d.
The motor relay driving circuit 25 in the ECU 11 outputs the control signal CM to the motor relay 16 when the braking pressures are adjusted and drives the motor 15 by turning on the normally open contact 16a by energizing the coil 16b of the motor relay 16.
As shown in FIG. 15, a reservoir tank 14 is disposed to a portion of a circulating hydraulic passage between the motor 15 and the respective actuators 10a-10d in the vicinity of the motor 15 to supply and collect a hydraulic pressure to and from the respective actuators 10a-10d through the hydraulic passage.
When attention is paid to one of the actuators (for example, the actuator 10a) in FIG. 15, it is arranged as shown in FIG. 16.
In FIG. 16, the actuator 10a includes a pressure maintaining solenoid valve 12 and a pressure reducing solenoid valve 13 and the other not shown actuators 10b-10d have the same arrangement.
The pressure maintaining solenoid valve 12 is disposed to the inlet hydraulic passage from the master cylinder 9 to the brake unit 7a and the pressure reducing solenoid valve 13 is disposed to the outlet hydraulic passage from the brake unit 7a to the reservoir tank 14.
That is, the pressure reducing solenoid valve 13 is disposed to the liquid pressure collecting passage from the reservoir tank 14 to the master cylinder 9 through the motor 15 for supplying and collecting the liquid pressure.
With this arrangement, the respective solenoid values 12, 13 are energized or deenergized in response to the control signal Ca from the ECU 11 to thereby switch the maintenance, the increase and the reduction of the braking pressure.
Ordinarily, the pressure maintaining solenoid valve 12 is opened and the pressure reducing solenoid valve 13 is closed.
In FIG. 16, when the driver depresses the brake pedal 8, a pressure is supplied to the master cylinder 9 and the braking fluid fed from the master cylinder 9 flows into the braking unit 7a through the pressure maintaining solenoid valve 12 in the actuator 10a to thereby increase the braking pressure Pa.
When a wheel deceleration corresponding to a locked state is detected and the control signal Ca indicating pressure reduction is created by the ECU 11, the electromagnetic solenoids of the pressure maintaining solenoid valve 12 and the pressure reducing solenoid valve 13 are driven by being energized.
At the time, the pressure maintaining solenoid valve 12 is closed to thereby shut off the hydraulic passage from the master cylinder 9 to the brake unit 7a.
Further, the pressure reducing solenoid valve 13 is opened to thereby connect the hydraulic passage from the braking unit 7a to the reservoir tank 14.
Therefore, the braking fluid in the brake unit 7a flows into the reservoir tank 14 and the braking pressure Pa is reduced.
At the same time, since the ECU 11 creates the control signal CM for energizing the motor relay 16 and operates the motor 15, the pressure of the braking fluid having flown into the reservoir tank 14 is increased and the braking fluid having the increased pressure is returned to the main passage on the master cylinder 9 side to be used in the next brake control.
Thereafter, when the ECU 11 creates the control signal Ca for maintaining pressure and only the pressure maintaining solenoid valve 12 is energized (the passage is closed), since the other valves are deenergized, all the hydraulic passages are shut off and the braking pressure Pa to the wheel 1a is maintained.
When the ECU 11 creates the control signal Ca for increasing pressure and the pressure maintaining solenoid valve 12 and the pressure reducing solenoid valve 13 are deenergized, the hydraulic passage between the master cylinder 9 and the brake unit 7a is connected again.
With this operation, since the high pressure braking fluid having been returned to the main passage on the master cylinder 9 side flows into the brake unit 7a again together with the braking fluid discharged from the motor 15, the braking pressure Pa to the wheel 1a is increased.
FIG. 17 is a timing chart showing the above antiskid brake control operation. The timing chart shows the change in time of each of the wheel speed Vwa calculated from the wheel speed signal Va, the wheel deceleration Gwa and the braking pressure Pa, and what is shown here is a case that the braking pressure Pa is relatively preferably adjusted.
In the drawing, the abscissa shows a time t and it is assumed that the wheel deceleration Gwa in a downward direction (negative acceleration) is in a positive direction and the braking pressure (braking hydraulic pressure) Pa in an upward direction is in an pressure increasing direction.
In FIG. 17, a basic vehicle speed Vr (refer to a dot-dash-line) is determined based on the wheel speed Vwa and a threshold value A relating to the wheel deceleration Gwa (refer to a dot-dash-line) is determined based on the maximum value of a road surface friction factor .mu. (peak value .mu.P).
The road surface friction factor .mu. is presumed based on the change in time (inclination) of the waveform of the wheel speed Vwa.
First, when the driver depresses the brake pedal 8 at a time t1, the level of the wheel speed Vwa is reduced by the increase of the braking pressure Pa.
At the time, when the wheel deceleration Gwa exceeds the threshold value A corresponding to a locked state as shown by the slant lines in FIG. 17, the braking pressure Pa is reduced to thereby prevent the occurrence of the locked state beforehand.
That is, since the braking pressure Pa is maximized at a time t2 when the wheel deceleration Gwa exceeds the threshold value A as well as a slip larger than a predetermined amount occurs, it is reduced by the commencement of antiskid brake control.
Thereafter, the braking pressure Pa is maintained to a constant value from a time t3 and the waveform of the wheel speed Vwa approaches the basic wheel speed Vr during the time.
The braking pressure Pa starts to be increased at a time t4 when the wheel deceleration Gwa is made equal to or smaller than the threshold value A as well as the slip is also made equal to or smaller than the predetermined amount and the increase of the pressure Pa is continued until a time t5 when the wheel deceleration Gwa exceeds the threshold value A.
As shown by the slant lines in FIG. 17, when the wheel deceleration Gwa exceeds the threshold value A at the time t5 again, the braking pressure Pa is reduced likewise and thereafter the occurrence of the locked state is prevented beforehand by repeating the same operation as above.
To realize the ideal antiskid brake control as shown in FIG. 17, for example, the threshold value A must be set larger than the maximum value of the road surface friction factor .mu..
The antiskid brake control is also executed to the other wheels 1b-1d in the same manner as above.
As described above, the locked state of the wheel 1a can be avoided by adjusting the braking pressure Pa by repeating the reduction, maintenance and increase of the braking pressure Pa in response to the control signal Ca from the ECU 11.
Incidentally, the threshold value A must be properly set in correspondence to the road surface friction factor .mu. as the antiskid control condition of the braking pressure Pa as described above. For this purpose, it is preferable to presume the road surface friction factor .mu. based on the basic vehicle speed Vr calculated from the wheel speed and to change the threshold value A of the wheel deceleration Gwa in accordance with the presumed road surface friction factor .mu..
However, the conventional antiskid controllers set the threshold value A as a condition for adjusting the braking pressure Pa constant regardless of the road surface friction factor .mu.. Thus, if the road surface friction factor .mu. is low, since the braking pressure Pa cannot be reduced until the wheel deceleration Gwa is greatly reduced, the occurrence of the locked state cannot be prevented.
Whereas, when the road surface friction factor .mu. is high, since the braking pressure Pa is reduced to a degree larger than necessary, a braking capability is reduced and a long time is required until the vehicle stops.
To solve the above problem, the controller disclosed in, for example, Japanese Unexamined Patent Publication No. 8-239024 is arrange such that the peak value of the road surface friction factor .mu., that is, a maximum road surface friction factor .mu.P is set as the threshold value A and the braking pressure Pa is reduced based on the difference between the threshold value A (.dbd..mu.P) and the wheel deceleration Gw.
However, it is very difficult to presume the maximum road surface friction factor .mu.P because a road surface ceaselessly changes as the vehicle travels.
Further, since the wheel deceleration Gw is liable to be made to a fluctuating waveform by the effect of a disturbance noise (the vibration of the vehicle) which occurs when the wheel speed Vw is detected and further the magnitude of the disturbance noise changes in accordance with the road surface friction factor .mu., a braking capability is made insufficient by the execution of useless pressure reduction.
FIG. 18 is a timing chart showing the change in time of the braking pressure Pa when the road surface friction factor .mu. is high or when a road is bad with many irregularities.
In FIG. 18, the wheel deceleration Gw fluctuates in the vicinity of the maximum road surface friction factor .mu.P due to the disturbance noise occurred when the vehicle travels and the threshold value A (refer to a broken line) is set to a level larger than the maximum road surface friction factor .mu.P (refer to a dot-dash line).
In general, the wheel deceleration Gw is liable to fluctuate in the vicinity of the maximum road surface friction factor .mu.P as shown in FIG. 18 by the resonation caused by the tires of the vehicle and the spring members thereof such as suspensions and the like and the rigidity component of the mounting parts of the wheel speed sensors 2a-2d and the brake units 7a-7d (refer to FIG. 14-FIG. 16).
In this case, when the maximum road surface friction factor .mu.P is set as the threshold value, since the braking pressure is reduced to a degree larger than necessary each time the wheel deceleration Gw exceeds the maximum road surface friction factor .mu.P, a sufficient braking capability cannot be obtained.
To prevent the above problem, the threshold value A is set to a level which is higher than the maximum road surface friction factor .mu.P by a value corresponding to a disturbance noise component.
However, as apparent from FIG. 18, since a short pressure reducing period T is repeated due to the fluctuation of the wheel deceleration Gw, the braking pressure P cannot be sufficiently reduced.
Further, a pressure reduction cannot be executed in correspondence to a road whose surface changes from a state that the maximum road surface friction factor .mu.P is large to a state that the maximum road surface friction factor .mu.P is small.
In addition, although the maximum road surface friction factor .mu.P is to be presumed from the wheel speed Vw, actually it is difficult to presume the maximum road surface friction factor .mu.P and what can be made at the best is to detect an average road surface friction factor suitable to a road surface at a time.
As described above, since it is not only difficult for the conventional antiskid brake controllers to detect or presumingly calculate the maximum road surface friction factor .mu.P correctly but also the braking pressure is reduced based on the constant threshold value A, the threshold value A cannot be suitably set in correspondence to the road surface friction factor .mu.. Thus, there is a problem that an optimum lock avoiding control suitable to a road surface state cannot be realized without sacrificing a braking capability.
An object of the present invention is to solve the above problems by providing an antiskid brake controller capable of securing a sufficient braking capability as well as securely avoiding a locked state by presuming a road surface friction factor based on a basic vehicle speed calculated from wheel speeds and setting a proper threshold value in accordance with a road surface state.
Another object of the present invention is to provide an antiskid brake controller capable of securing a sufficient braking capability as well as securely avoiding a locked state by setting a plurality of smoothed slip characteristics, a plurality of smoothed wheel deceleration characteristics and a plurality of threshold values in accordance with a magnitude of a slip as braking pressure adjusting conditions.